In man-machine communication there is a trend away from the use of mechanical keys and towards the use of touch-sensitive keys. Touch-sensitive keys have several advantages compared to mechanical keys; they have no moving parts and are thus not subjected to wear, they can more easily be sealed hermetically from the environment, they can be manufactured at a lower cost, and designs with touch-sensitive keys often offer aesthetically more pleasing solutions.
Several concepts for optical touch-sensitive keys are known in the prior art. In IBM Technical Disclosure Bulletin Vol. 18 No. 2 page 483 (July 1975) a fibre-optic switch is described in which the pressure of the finger deforms a foam washer in such a way that reflected light from an optical fibre is prevented from reaching a return fibre.
In Electronic Engineering, April 1981, page 51, several designs are presented in which an approaching finger either reflects or obstructs the light path from a source fibre to a detector fibre. No consideration is given in the mentioned reference to the serious problem of external light, such as from the sun, shining into the system.
None of the above prior art references, however, relates to a true touch-sensitive key in the sense that its function involves no mechanical deformation and that it is activated when and only when the finger touches the sensitive surface.
British Pat. No. 1,600,566 (corresponding to U.S. Pat. No. 4,254,333 and corresponding Patents in other countries) describes a concept for a true optical touch-sensitive key, in which also a complete separation is achieved between, on one hand, external light such as from the sun and, on the other hand, an inner radiation system operable to affect a radiation detector. Despite the fact that said inner radiation system is completely separated from the outside, it is still possible, due to the particular properties of total internal reflection, to affect this inner radiation system by touching the surface of the key. The present invention is concerned with a development of this concept which is particularly suitable for use in a fibre-optic context bearing in mind that an important requirement with a fibre-optic touch-sensitive key is that the light loss in the key should be as small as possible, so as to avoid the need for amplification in connection with the key requiring extra electronic circuitry which would be expensive and also might make the system sensitive to external electromagnetic interference.
One way of reducing light losses would be to focus the light emitted by an input fibre onto an output fibre by means of lenses in front of the input and output fibres, respectively. However, this has the disadvantage that it tends to lead to clumsy designs and also is not ideal with respect to focusing deficiencies leading to light loss. Such deficiencies are inherent in the focusing process and are determined by the ratio of the size of the source to the size of the image-forming surface. For this reason the present invention is mainly concerned with the use of larger image-forming surfaces, such as the touching surface itself. In this sense the present invention displays a superficial resemblance with Derwent's abstract No. H8843 B/37, SU 636-803. This reference, however, relates to a surface of a rotating logarithmic spiral which surface has inferior focusing properties. Said reference also makes no provision for discrimination against ambient light, such as from the sun, which can shine directly onto the photoelectric sensor and cause severe malfunction.